Re: Parentheses in FROM clause and evaluation order. - Mailing list pgsql-sql

Hi,

The final result seems to be the same, I just was curious about the
standard behavior. Does the SQl says something about this execution
order?

Thanks for your response.

salu2
dario estepario ...


2005/8/15, Stephan Szabo <[email protected]>:
> On Wed, 10 Aug 2005 [email protected] wrote:
>
> > I thought that the parenthesis in the table expression
> > (FROM clause), could be used to indicate the desired
> > evaluation order. But, I tried with a couple of samples
> > and the explain command returned me the same result; no matter
> > what parentheses association I used. I am using only INNER JOINs.
> >
> > In fact, I thought that the whole table expression was gonna be
> > evaluated before the WHERE filter. Does the stantard says something
> > about this evaluation order when the parentheses are present?
> > Does PostgreSQL implements this behavior?
>
> AFAIK we only try to provide final results that are equivalent to
> following the steps in order, so it'll reorder joins or push clauses
> around as long as it thinks the semantics of the query won't change. For
> example, actually doing unconstrainted joins before where clauses is a
> very bad plan if you've got a FROM table1, table2, table3 style query. If
> you're seeing a place where the reorder affects the query results as
> opposed to the query plan, that's probably a bug, can you give more
> information?
>